Disk drive magnetic recording is accomplished by the relative motion between a magnetic head and a magnetic medium. In a disk drive system, the magnetic medium typically comprises a circular rigid disk having a magnetic material incorporated or embedded into its surface. As the disk is rotated, a hydrodynamic air film develops between the magnetic head and the surface of the magnetic medium. Thus, the magnetic head is said to "fly" above the surface of the disk. In state-of-the-art disk drive units, flying heights are on the order of one microinch above the surface of the disk.
The low flying heights commonly found in modern disk drive systems mandate that the magnetic disks themselves be manufactured to have ultra-smooth surfaces. Unfortunately, during the manufacturing process, the magnetic recording surface may be inadvertently damaged. Alternatively, contaminate particles introduced in the manufacturing process can end up causing imperfections on the disk surface. These imperfections can adversely affect the way the head flies. For example, a rough surface texture with numerous defects projecting above the planar surface of the disk can cause destructive head crashes. Conversely, pits or voids in the recording surface may result in a loss of magnetic information--even without interfering with the flying characteristics of the slider or head.
Because of the critical requirement for an ultra-smooth disk surface, manufacturers of magnetic recording disks must devote considerable time to certifying the surface characteristics of the magnetic disk. Typically, this involves a visual inspection of the disk's surface for defects. It is desirable to both qualify and quantify the type, as well as the location, of imperfections present on the surface of the recording medium. To avoid problem areas during recording, there is also a need to map the entire disk surface so as to be able to locate defects and identify their type. If the number and/or type of defects exceeds acceptable tolerances, the entire disk may be rejected. By way of example, if a particular defect is very large and protrudes above the surface of the disk, the disk might be rejected.
As will be seen, the present invention provides an apparatus and method for optically inspecting the surface of magnetic disks. According to the invention, defects and problem areas are pinpointed with an optical detection apparatus capable of differentiating between various types of defects. The invention also satisfies the need for an inspection tool that provides both qualitative and quantitative analysis of the disk's surface. The inspection apparatus is highly accurate and provides a simple, cost-effective method for certifying magnetic disks.